This invention relates to a system for controlling a motor driven function by sensing the input current to the motor, and more particularly controlling the pressure of a pump by sensing the input current to a permanent magnet (PM) brushless motor.
Presently, many fluid pumps, including air compressors, use series wound (universal) brush motors that are powered directly from an AC power line. While inexpensive, these motors have certain drawbacks. Among these are brush wear which can limit the useful life of the equipment, and sparking of the commutator, which may be a hazard in certain environments. The pressure in these pump systems is controlled by energizing a motor when the pressure drops below a set point threshold and de-energizing the motor when it rises above the set point threshold. A pressure sensor, either a mechanical type that directly operates a motor energizing switch or a strain gauge type that electronically controls such a switch, regulates the pressure. Brushless DC motors have not been commonly known in these applications, due to the extra cost of the motor controller which is necessary to control such a motor.
The invention is practiced in a method and apparatus for controlling the output function of a permanent magnet brushless DC motor, by sensing an input current to the motor, by computing an output torque generated by the motor as a function of the input current to the motor, by computing an output pressure for a pump in response to the output torque, by reading a set point pressure, and by comparing the set point pressure to the output pressure, and in response thereto, controlling on-off operation of the motor.
The invention provides for estimating the output pressure from input current, which allows for control of a pump without the use of a pressure sensor.